1. Field of the Invention
The present invention relates to curable compositions for preparing antistatic transparent abrasion resistant coatings and articles exhibiting antistatic and abrasion resistance properties coated therewith.
The invention also relates to articles, especially optical and ophthalmic lenses for eyeglasses comprising at least one antistatic coating obtained by depositing and curing a curable coating according to the invention.
2. Description of Related Art
It is well known that optical articles, which are composed of essentially insulating materials, have a tendency to get charged with static electricity, especially when they are cleaned in dry conditions by rubbing their surface with a cloth or synthetic piece, for example a polyester piece (triboelectricity). The charges which are present at the surface of said optical articles create an electrostatic field capable of attracting and fixing, as long as the charge remains on optical articles, objects lying in the vicinity thereof (a few centimeters) that have a very little weight, generally small size particles such as dusts.
In order to decrease or suppress attraction of the particles, it is necessary to decrease the intensity of the electrostatic field, i.e. to decrease the number of static charges which are present at the surface of the article. This may be carried out by imparting mobility to the charges, for instance by introducing in the optical article a layer of a material inducing a high mobility of the charges. Materials inducing the highest mobility are conductive materials. Thus, a material having a high conductivity allows dissipating more rapidly charges.
By “antistatic”, it is meant the property of not retaining and/or developing an appreciable electrostatic charge. An article is generally considered to have acceptable antistatic properties when it does not attract or fix dust or small particles after one of its surfaces has been rubbed with an appropriate cloth. It is capable of quickly dissipating accumulated electrostatic charges.
This property if often related to the static potential of the material. When the static potential of the material (measured when the article has not been charged) is 0 KV+/−0.1 KV (in absolute value), the material is antistatic. On the contrary, when its static potential is different from 0 KV+/−0.1 KV (in absolute value), the material is said static.
The ability of a glass to evacuate a static charge created by rubbing with a cloth or any other electrostatic charge generation process (charge applied by corona . . . ) can be quantified by measuring the time required for said charge to be dissipated (charge decay time). Thus, antistatic glasses have a discharge time in the order of 100-200 milliseconds, while static glasses have a discharge time in the order of several tenths seconds, sometimes even several minutes. A static glass having just been rubbed can thus attract surrounding dusts as long as it requires time to get discharged.
It is known in the art to prepare conductive inorganic and organic layers for anti-static applications.
It is known to achieve high optical transparency in anti-static coatings (over 90% or even 95% transmittance in the visible light) by using vacuum-deposited ITO layers.
However, the performance of ITO is affected when applied to plastic. These thin coatings are fragile and are readily damaged during bending or other stress inducing conditions.
Coatings comprising conducting polymers such as Baytron P® are also known as being able to impart antistatic properties.
The use of Baytron P® is known in different commercial applications such as photographic films, electronics packaging and imaging materials.
However, Baytron P® antistatic hard coatings with overall excellent performances, including high transmittance, low haze, and excellent adhesion and abrasion resistance, have been barely addressed in prior art, especially in ophthalmic lens field.
U.S. Pat. No. 6,663,956 to Heberger et al describes that Baytron PH®-based antistatic coatings show adhesion and high transmittance by optimizing the concentration of surfactants and nanoparticles. A high level of surfactant is used. The coating solutions as well as the coatings comprise low level of polymeric binders. The resulting coatings still present some haze.
This antistatic coating is preferably not overcoated with another coating. Such a top coating could limit the ability of the antistatic coating to prevent static effects.
U.S. Pat. No. 6,479,228 to Majumdar also discloses transparent Baytron P®-based coatings exhibiting some scratch resistance and antistatic properties, but the Taber Ahaze value is not fully satisfying.
U.S. Pat. No. 6,211,274 describes coating compositions comprising a composite colloid which is prepared by mixing a conductive oxide such a conductive zinc antimonate and a conductive polymer such as Baytron P®. The inorganic-organic composite conductive sol of the conductive oxide and the conductive polymer thus obtained has a particle size of 100 to 300 nm, measured by the laser scattering method.
The polythiophene colloids are adsorbed on or bonded to the periphery of the anhydrous zinc antimonate. Practically, the size of the composite particles between is between 151 and 193 nm.
This size of particles is quite high for an application in the optical field, especially in the ophthalmic field, and particularly for eyeglasses and may lead to articles exhibiting a certain level of haze non acceptable in the ophthalmic field.
U.S. Pat. No. 6,084,040 discloses a scratch resistant conductive coating of polythiophene salts wherein silanes have been hydrolyzed in the presence of conductive polythiophene salts and the use thereof for the production of scratch-resistant electrically conductive coatings.
Tetraalkoxysilanes and alkyl or aryltrialkoxysilanes are used as silanes.
Fillers, for example silicon dioxide, such as colloidal silicon dioxide, titanium dioxide and zinc oxide, can be added to the coatings and form transparent coatings.
The coatings can also comprise additives improving adhesion to the respective substrate, such as epoxysilanes and γ-glycidoxypropyltrimethoxysilane.
However, none of the examples of this patent describes compositions containing fillers.
γ-glycidoxypropropyl trimethoxysilane is merely used as an optional component. The components that have to be used in the conductive coatings of U.S. Pat. No. 6,084,040 are tetra-alkoxysilanes and alkyl or aryl trialkoxysilanes that do not contain epoxy groups. Such components are generally used as the main components of the conductive coating composition.
This document does not mention any reference to the use in ophthalmic articles such as eyeglass lenses.
As seen above, the antistatic hard coatings of the prior art still have certain limitations preventing them from some specific applications, especially in the ophthalmic lens application.
Therefore, conductive coatings providing antistatic properties, having low haze and excellent hardness and/or abrasion resistance at the same time are very desirable in these specific applications.
There is still a need of coating compositions which can retain their antistatic properties inside a stack of several layers, including inorganic dielectric layers deposited thereon.